Physics and math question please loook and hhhhheeeeeelllllpppppp ?

Question

. A train is traveling down a straight track at 21 m/s when the engineer applied the brakes, resulting in an acceleration of -1.0 m/s2 as long as the train is in motion. How far does the train move during a 39 s time interval starting at the instant the brakes are applied?

Answer 1

You know that

v0 = 21 m/s

a = -1 m/s^2

t = 39 s

HOWEVER!  If you start at v0 = 21m/s and a=-1 m/s^2, how long does it take to stop?

v = a*t

t = v/a

t = (21m/s) / (1m/s^2) = 21 s

so

x = v0*t + (1/2)*a*t^2

x = (21m/s)*(21s) + (1/2)*(-1m/s^2)*(21s)^2

x = 220.5m

Answer 2

210 m.